5o6 MOSSES AND FERNS  chap. 



" primary " and " secondary " prothallium is, as we have seen, 

 only apparent, and the diaphragm in the prothalHum of 

 Selaginella is not a true cell wall, marking a primary division 

 of the spore contents, but only a secondary thickening of the 

 lower walls of certain cells, indicating a temporary cessation in 

 the process of cell-formation. It is by no means improbable 

 that this cell-formation may sometimes go on uninterruptedly, 

 in which case no diaphragm would be formed, and, as in Isoetes, 

 there would be no distinct line of demarcation between the 

 archegonial tissue at the apex and the large-celled nutritive 

 tissue below. 



The presence of a suspensor in all investigated Lycopodineae 

 is a character which distinguishes them at once from the other 

 Pteridophytes, and has its closest analogy again among the 

 Conifers. 



Fossil Lycopodine(Z ^ 



Many fossil remains of plants undoubtedly belonging to 

 the Lycopodineae are met with, especially in the Coal-measures, 

 where the Lepidodendreae were especially well developed. Of 

 homosporous forms, it seems pretty certain that the fossils 

 described under the name Lycopodites are related to the living 

 genus Lycopodium, and certain fossils from the Coal-measures 

 have even been referred to the latter genus, some of these 

 being homophyllous, others heterophyllous. Solms-Laubach 

 thinks it somewhat doubtful whether the plants described by 

 various writers, and belonging to older formations, really are 

 Lycopodineae. 



In regard to the Psilotace^ he says: "The statements 

 respecting fossil remains of the family PsilotacecB are few and 

 uncertain, nor is this surprising in such simple and slightly 

 differentiated forms. If Psilotites . . . does really belong to 

 this group, a point which I am unable to determine from the 

 figures, we should be able to follow the type as far down as 

 the period of the Coal-measures." 



The genus Psilophyton, which has been found as far back 

 as the Upper Silurian, is regarded by Dawson ^ as related to the 

 Psilotaceae, but there seems some question about the accuracy 

 of his conclusions. 



The well-known group of the Lepidodendreje is one of the 



1 Solms-Laubach (2). - Solms-Laubach (i), p. 190. 



